Creating an emission texture image from a "normal" image
Here's one possible way to do what you want, if I understand your request properly.



The mix shader on the lower left is just the one that comes from the Images as Planes addon. That could just as easily be the direct Color out of the Image node. What I'm doing here is crushing the darker areas so that the image is mostly just the lights and the skyline. This goes into an Emission shader which, by use of the Is Camera Ray output as the Mix Shader factor, lights the scene but doesn't change the appearance of the image.



If this isn't what you're looking for, please update your question to give more information.